Maybe some of you guys don't remember how we got here. There were so many shenanigans going on in the 1970s wrt power output claims that the Federal Trade Commission felt it necessary to clamp down and impose a defined standard and test procedure that people could use to compare products reliably.
The standard was called root mean square. It was based on measuring the average (or mean) continuous voltage times the average amperage (square) of an amp reproducing a broadband signal (typically white or pink noise), before applying the root to yield value.
RMS is a recognized abbreviation. Wattage ratings in RMS (which I will refer to as WRMS for brevity) continues to be a recognized CE standard. There is a great deal of semantic debate elsewhere whether this is an appropriate term. Regardless, it is an accurate description of the calculation method used.
The FTC procedure used to measure WRMS involved preconditioning an amp for 30 minutes so that it would be at its normal operating temperature.
This was a tough procedure that many amps could not pass to the satisfaction of manufacturers. So a number of companies got together to form the Institute for High Fidelity (IHF) and develop a competing standard. This standard was less strenuous because it was based on a 20 millisecond burst, the peak output of which was measured and expressed in watts.
I'm not aware of any specified preconditioning time associated with the IHF or peak standard. Since this can play both ways (depending on design),I suspect manufacturers use whatever preconditioning method yields the best figures. How this aligns with average use is anyone's guess.
With a continuous broadband signal, the resulting measurement is representative of the total power occurring under the curve. With a burst signal, it only reflects the power over a 20 millisecond duration. That's 1/50th of a second, folks.
To achieve a certain RMS rating, manufacturers must ensure that the power stage of the amp is continuously supplied. Large storage capacitors can be used, but they're unable of providing energy continuously.... they can discharge completely and this has an impact on their longevity. So a pretty robust transformer is needed. That's why manufacturers of such gear often list a current output rating. It's hard to achieve current delivery in the 30-60 amp range but well worth it because this is what allows amps to operate loudspeakers down to 4 ohms.
This is not the case with an amp rated using IHF or peak power. Manufacturers are, therefore, able to use smaller transformers and just enough capacitors to achieve a certain peak amplitude over 20/1000ths (or 1/50th) of a second.
Amps rated under the RMS system (and especially those rated in "all channels driven") are normally quite capable of driving difficult loads, such as occur when speaker capacitance drops to 4 ohms or below.
Amps rated under an IHF or peak power, even when rated "all channels driven", can't do this without clipping. Why? Because doing so would require outputs of up to 50 times longer than what they were designed for (20 ms --> 1000 ms).
To mitigate the risk of clipping, some manufacturers employ anti clipping circuitry to suppress power output. Others rate their amps for "two channels" driven, knowing full well that their transformers are incapable of delivering the rated power to all channels at the same time.
Now some may say that the 1970s were a long time ago. Let me assure you that cost/profit models haven't changed over 40 years. Good, robust transformers cost money to build and money to ship. AB amps capable of high continuous output need big heat sinks and cooling fins. These cost money to design, build, and ship.
The issues haven't changed either, despite what those selling cheap multichannel amps would like you to think. There are qualitative and quantitative differences between an amp capable of continuously providing "x" watts versus one that can deliver the same wattage for a fraction of that time.
If you want an independent way to check the truth, do the following:
1. Look up the manufacturer's figure for whatever power (in watts) your amp is rated to consume (I.e. Power from the wall socket);
2. For an AB amp, divide that figure by 2 (AB amps are slightly more efficient than 50%, but some power is required to drive the preamp, led/LCD displays, equalization circuits etc.)
3. Take the sum of #2 and divide it by the total number of channels of your amp. The result is the theoretical maximum continuous power output of your amp/channel. I say theoretical because the real continuous output is often less.
This is basic electrical theory. Amps don't generate power, they are net users of power as they convert electrical energy from one form into another. So continuous power going into the amp MUST be the same or greater than power leaving the amp.
All this means that a 2 channel AB amp rated to draw 1800W of power (15A, the max load of a standard socket in North America, @ 120V) cannot produce more than 450WRMS per channel.
A 5 channel AVR drawing the same power cannot produce more than 180WRMS/channel.
A 7 channel AVR drawing the same power cannot produce more than 128 WRMS/channel.
The actual ability of an AB amp to reach these levels is entirely dependent on its design. No heavy heat sinks or cooling fins/fans makes it impossible for the unit to shed the 215 calories of heat per second (which is what 900W of energy, or half the 1800W load equates to in heat) to prevent the amp's temperature from running out of control. In comparative terms, this is about the same heat generated as a hair dryer operating at a "medium" heat setting.
Now if your AB amp draws something more realistic like 1200W (or 10 amps @ 120V),then its theoretical maximum continuous power output and thermal load would be 1/3 less than the figures above (2 Ch: 300WRMS/channel, 5 Ch: 120WRMS/channel, 7 Ch: 85WRMS/channel).
This is a lot to take in... and I do apologise for the length of this post. Not everyone will agree with my conclusions. But it is basic electrical theory and simple math, along with historical references, which you can verify for yourselves.
